1. Field of the Invention
The present invention relates to non-volatile memories, particularly electrically erasable and programmable memories of the EEPROM and Flash type.
2. Description of the Related Art
The use of electrically erasable and programmable non-volatile memories has become widespread in recent years, and such memories can be found in many integrated circuits. In certain applications, it is sometimes desired for it to be possible to erase the content of a non-volatile memory in a very short space of time when an unexpected or prohibited event occurs. Thus, integrated circuits for smart cards are equipped with memories of the aforementioned type in which confidential information is stored such as cryptography codes, passwords, identification data, etc. When an unauthorized read attempt occurs, provision is made to rapidly destroy the confidential information.
The destruction of the content of a non-volatile memory generally involves erasing the entire memory array. However, this solution has the disadvantage of taking a long time to implement, particularly in word-erasable EEPROM memories, as the memory array is erased word by word.
Thus, the present invention aims to provide a method enabling the content of a non-volatile memory to be rendered unavailable, without erasing the memory cells.
To this end, the present invention proposes providing a means for distorting data read during a reading of the data present in the memory rather than a destruction as such of said data. Such distortion can be reversible or irreversible.
It is well known that the memory array of a non-volatile memory includes memory cells connected to word lines and to bit lines. The bit lines are linked to sense amplifiers, and the value of the data read by the sense amplifiers in the memory cells varies according to the on or off state of the memory cells at the moment at which a read voltage is applied to them. The on or off state of a memory cell itself varies according to the programmed or erased state into which the memory cell has been put.